This invention relates to heat removal devices and, more particularly, to heat removal devices which maintain preselected temperature conditions under variable heat input loads.
There are many applications where a constant temperature must be maintained through changes in operating conditions and heat removal requirements. One such device is a radio-frequency quadrupole (RFQ) accelerator in the production of high-energy particle beams. The basic function of the RFQ accelerator is to combine the focusing characteristics of a chain of alternatively rotated quadrupole lenses with the accelerating characteristics of a chain of rf cavities of graded spacing, converting a moderately diverging low-energy input beam into a well-collimated high-energy output beam. A typical RFQ accelerator structure is a quasi-cylindrical hollow steel shell, the inside of which is fitted with four copper-clad longitudinal vanes spaced 90.degree. apart and extending radially inward from the interior shell wall. Longitudinal bore holes may be provided through which water or other fluids are circulated to remove from the Vanes and shell the excess heal generated by residual beam losses and large rf currents flowing in the skin-depth layer of the interiorly facing metal walls. One conventional RFQ accelerator operates with an rf driving power of up to 530 kW. much of Which must be removed as heat.
Conventionally, the four RFQ vanes extend radially inward almost to the central Z-axis of symmetry, with the adjacent vane edges remaining separated from each other by a small gap, which is typically a few millimeters. In order to produce the required acceleration of the beam particles, the inner edge of each RFQ vane is smoothly serrated in the longitudinal direction, approximating a sine wave of gradually increasing wavelength in the +.z direction, i.e., in the direction of beam travel. In the x- and y-planes, the transverse valleys in the two opposing vane edges are positioned opposite each other in longitudinal agreement. However, the x-plane valleys are opposite the y-plane hills in the final assembly.
In view of the precise vane edge alignments which are required, it is apparent that the RFQ accelerator cannot operate effectively with uncompensated temperature changes. For example, the thermal expansion coefficient is 9.4.times.10.sup.-6 per .degree. F. for copper and 6.7.times.10.sup.31 6 per .degree. F for steel. Given the narrow channel needed for strong electric fields across the beam, even small changes in temperature will change the intervane capacitance and significantly detune the cavities represented by the RFQ quadrants.
By way of example, in an RFQ accelerator having a Q-value of 8.500 and a resonant frequency of 425 MHz at 80.degree. F., the corresponding half-power bandwidth is 50 kHz. and the thermal resonance detuning coefficient is -3.3 kHz per .degree. F. The amplitude and phase responses at the operating frequency will change accordingly as the resonant frequency shifts from the operating frequency. Even a 15.degree. F. temperature deviation will reduce the amplitude response by 55% and shift the phase response by 63.degree..
One of the vane-edge design equations requires the serration wavelength to increase along the beam axis at a rate which is inversely proportional to the square of the product of the resonant frequency and the gap span. However, both the resonant frequency and the gap span vary inversely with temperature. Consequently, a simple retuning of the klystron driver frequency to compensate for a temperature-induced change in resonance will result in an undesirable departure from the spatio-temporal coherence needed for optimum operation of the RFQ accelerator. Retuning of the klystron driver also makes a combined RFQ/DTL (radio frequency quadrupole/drift tube linac) operation difficult, since the RFQ and DTL units generally have different responses to changes in beam loads and temperature.
The use of motorized tuning slugs or similarly movable internal elements to compensate for temperature changes in the RFQ accelerator results in undesirable disturbances of the electric fields needed across the vane gaps. Accordingly, it is seen that temperature stabilization of the RFQ resonant frequency is required. It would be desirable to obtain temperature stabilization by control of the temperature of the coolant circulating through the vane bore holes.
The temperature control problem is addressed by the present invention and improved methods and apparatus are provided for accurately stabilizing the temperature of a device under selected transient conditions.
Accordingly, an object of the present invention is to provide a system which will stably and accurately control the temperature of a remote thermal unit which is subject to unpredictable heat loads.
Another object of the present invention is to provide a method for controlling the temperature of a randomly perturbed remote thermal unit by diluting the flow of a main coolant fluid with a second coolant fluid of substantially lower temperature.
Still another object is to provide a stable and accurate temperature control system which automatically tunes the resonant frequency of a radio-frequency quadrupole accelerator to the fixed frequency of its UHF (ultra-high-frequency) driver, and maintains its resonant condition irrespective of unpredictable changes in the RFQ heat load.
One other object is to maintain a stable temperature in a device which is thermally remote from the temperature control system, wherein both thermal and transport lags must be considered.